Piston and method of making same



1962 R. E. WILCOXON 3,070,414

PISTON AND METHOD OF MAKING SAME Filed Aug. 3, 1959 2 Sheets-Sheet 1 EgLZ fnz E'n/UF ea/p/i [Mk-axon 15y w; @Aa, 51x95.

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3,070,434 PISTQN AND METHOD F MAKHNG SAME Ralph E. Wileoxon, Eueiid, Ghio, assignor to Thompson Ramo Wooldridge Inc, Cleveland, Ohio, a corporation of Ohio Filed Aug. 3, 1959, Set. N 831,366 9 tllaims. (1. 309-) This invention relates to resilient pistons adapted to be tightly fitted in liquid cooled internal combustion engine cylinders and conform to the cylinder shape at engine operating temperatures and stresses without seizure or scufiing and without the control of heretofore necessary expansion control bands and struts. Specifically, this invention deals with oval skirt slot-type pistons composed of high silicon content normally brittle aluminum alloys and having extruded metal grain configurations accommodating reshaping of the skirt to conform with a cylinder wall during normal operation of the piston in an internal combustion engine thereby avoiding heretofore required expansion control bands and struts while accommodating sizing of the piston for the type of tight cold fits in ferrous metal cylinders that were heretofore limited for expansion control band or strut equipped pistons.

According to this invention, a relatively brittle aluminum alloy composed by weight essentially of from 9% to 24% silicon; from 1 /2% to 4% copper; from .2% to 1% manganese; and from .2% to 1% magnesium is cast into the shape of cylindrical slugs which are heated to forging temperatures of from 450 to 950 F. and are subjected to impact forging between dies and punches shaped for producing the desired piston configuration. The aluminum alloy flows by extrusion between the dies and punches to produce a grain band configuration which extends continuously across the head of the piston and thence through the ring belt, skirt and pin boss portions of the piston to terminate in the end edges of the skirt and define both the inner and outer faces of the head, the ring band, the pin bosses and the skirt with their sides only. The piston skirt is ground to oval shape with its major diameter normal to the wrist pin axis through the pin bosses, and this major diameter is sized to have a tight cold fit in the cylinder in which the piston is to be used of from .001 to .002 inch. Heretofore, such ti ht fits required expansion control bands or struts embedded in the piston for pulling an oval skirt into circular shape under the temperature conditions and stress conditions encountered in the engine. However, the grain formation and alloy composition of the pistons of this invention produce a resilient unit which will conform itself to the shape of the cylinder under the temperature and stress conditions encountered in operation, and these conditions cause the unit to take a permanent set which will not thereafter deform to vary the shape or clearance.

Prior to this invention bandless or strut-free aluminum pistons had to be very loosely fitted in cylinders to prevent subsequent seizure or scufiing. Loose fits of from .002 to .004 inch clearance at the major diameter of the piston skirt were required. Since tight fits, especially at the thrust or slap faces of the piston are highly desirable to control oil consumption and to maintain compression for good engine performance, it was necessary heretofore to use band and strut controlled pistons.

The pistons of this invention have suflicient resilience during initial operation in the engine so as to relieve stresses during the time a permanent set is being reached. The grain band configuration permits flexing at the tight areas which normally would seize before the permanent set is established. The metal flows from the major diameter toward the minor diameter Without the necessity of heretofore required control bands and struts.

The forged pistons of this invention are solution heat occurs at the temperatures normally encountered by pistons in internal combustion engine operations, and the pistons thereby automatically break-in in initial opera-v tion of the engine to produce non-seizing fits in this cylinders which have heretofore not been possible Without the aids of expensive expansion control inserts.

It is then an object of this invention to produce resilient aluminum pistons which will automatically conform themselves to the shape of the cylinders in which they operate at the operating temperatures and under the operating loads normally encountered in internal combustion engines.

A further object of this invention is to provide a band less and strut-free trunk-type aluminum piston having sufiicient resiliency to conform itself to the shape of a cylinder in which it operates and then take a permanent set in the cylinder under temperature and load conditions encountered in normal operation of the engine.

A still further object of this invention is to provide an extruded aluminum piston with a skirt of oval shape having a major diameter sized for tight cold fitting in a cylinder and possessing sufiicient inherent resilience to conform to the shape of the cylinder without seizing or scuffing.

Another object of this invention is to provide a bandless aluminum piston having a major diameter sized for fitting in a cylinder with clearance of the type heretofore reserved for insert controlled pistons.

Another object of this invention is to provide an oval shaped piston composed of a high silicon content aluminum alloy and adapted to take a permanent set while decreasing the major diameter thereof as it is subjected to temperatures varying from about F. in the skirt portion of about 550 peak loads of from 500 to 700 pounds per square inch such as occur during normal operation of an internal combustion engine.

A specific object of this invention is to provide an impact forged extruded aluminum piston having an oval shaped skirt with a major diameter normal to the pin boss axis of the piston which is sized for fitting in a cast iron cylinder having a diameter from .001 to .002 inch greater than said major diameter, said piston having a grain band formation accommodating deformation of the piston under load and temperature conditions encountered in normal operation of an internal combustion engine to prevent seizure at tight fitting areas of the piston skirt and cylinder.

Another object of the invention is to provide an aluminum piston which automatically breaks-in under normal operation in an internal combustion engine to conform with the cylinder of the engine and to assume a permanent set shape after being broken-in.

Other and further objects of this invention will be apparent to those skilled in this art from the following detailed description of the annexed sheets of drawings which, by way of preferred example only, illustrates one embodiment of the invention.

On the drawings:

FIGURE 1 is a longitudinal cross sectional view of a cast starting slug piece of aluminum for producing the pistons in accordance with this invention;

FIGURE 2 is an isometric view of a billet formed from the slug piece of FIGURE 1;

FIGURE 3 is a longitudinal cross sectional view of F. in the head portion and under 7 a piston blank formed according to this invention and showing the grain band formation of the blank;

FIGURE 4 is a view similar to FIGURE 3 but taken along the line IVIV of FIGURE 3;

FIGURE 5 is a plan view of the open end of the piston made from the blanks of FIGURES 3 and 4;

FIGURE 6 is a vertical cross sectional view of the finished piston taken along the line VIVI of FIG- URE 5;

FIGURE 7 is a diagrammatic view of the piston of FIGURE 6 seated in the bore of a cylinder showing the relative dimensions at a level in the head and ring hand area of the piston taken along the line VII-VII of FIGURE 6;

FIGURE 8 is a view similar to FIGURE 7 but illustrating the relative dimensions of the cylinder bore and piston skirt at the level of the piston along the line VIIIVIII of FIGURE 6;

FIGURE 9 is an enlarged diagrammatic view similar to FIGURE 8 but illustrating the relative dimensions of the cylinder bore and piston skirt after the piston has become permanently set;

FIGURE 10 is a side elevational view of the piston of FIGURE 6 showing the grain band formation and the manner in which the bands permit flow of metal around the skirt.

As shown on the drawings:

A cast aluminum cylindrical slug 10 shown in FIGURE 1 is provided to form the starting piece for the pistons of this invention. The slug 10 is composed of a relatively brittle aluminum alloy having very little ductility up to 400 F. Suitable alloys for forming the lugs are known commercially as Red X alloys, and the following composition is an illustration of a preferred suitable range of ingredients for the alloy:

Elements: Percent by weight Silicon 9 to 24 Copper 1 /2 to 4 Manganese .2 to 1 Magnesium .2 to 1 Aluminum Balance A preferred specific alloy has the following composition:

Impurities such as the following may be tolerated in the alloy:

Impurities: Percent by weight, maximum Iron 1.2 Zinc .4 Nickel .0 Titanium .2

As shown in FIGURE 1, the slug 10 has a cast crystalline metal structure, designated at 11, which is relatively free from grain bands or slip planes.

The slug 10 is initially kneaded into the form of the billet 12 shown in FIGURE 2 by impact forging the slug in a die with a punch at forging temperatures of from 450 to 950 F. The billet 12 is shorter in length but has a larger diameter than the slug 10, and as shown, has a cylindrical side wall 12a, a flat bottom 12b, a convex spherically domed top 120, a pair of fragmental cylindrical ears 12d, 12d radiating from the apex of the spherically domed top 12c, and a bevel 12e between the cylindrical side wall 12a and the flat bottom 12b. This billet 12 has a grain band or a slip plane structure developed from the crystalline structure 11 of the slug 10 by the kneading or impact forging operation.

In accordance with the second step of the method of forming the pistons of this invention, the billet 12 is subjected to an impace extrusion step between a die and a punch at the same forging temperatures used for forming the billet 12. The piston blank 13 illustrated in FIG- URES 3 and 4, formed by the second step of the method of this invention, is in the shape of a trunk-type piston having a head 13a with a depending ring flange or ring band 13b and a thinner skirt portion 130. Wrist pin bosses 13d, 13d extend from the head 13a on opposite sides of the ring flange and skirt to terminate in rounded ends inwardly from the open end of the skirt. These pins bosses are thicker than the skirt and the ring flange. The extremity of the skirt has a reduced thickness portion 132 which provides a large diameter cavity in the interior of the piston. The end wall 131 of the reduced diameter portion 13a is scalloped to provide recesses 13g, 13g on the wrist pin boss sides of the skirt. These recesses accommodate the crank shaft of an engine to permit close clearance relationship between the piston skirt and the crank shaft.

As shown in FIGURES 3 and 4, the piston blank 13 has slip planes or grain bands 14 extending continuously across the top of the head 13a and thence through the ring flange and skirt and through the pin bosses so that all exterior surfaces and interior surfaces of the piston will be defined by the sides of the grain bands. The grain flow is developed from the grain band formation in the starting billet 12. The grain bands 14, as shown, are substantially parallel to the inner and outer surfaces of the piston and these surfaces are relatively free from grain band ends. The resulting construction is strong, wearresistant and corrosion-resistant because the metal is worked in the direction of operation of the piston, and the surfaces will not have pits normally found after usage of pistons having grain band ends in their working surfaces.

To insure full forming of the skirt during extrusion, the ears 12d of th billet 12 are positioned in the forging die normal to slots in the punch which accommodate the flow of metal to form the wrist pin bosses 13d. The additional mass of metal provided by the cars will force extrusion through the thinner skirt portions 13: where flow of metal is normally restricted due to the relatively thin gap through which the metal must pass between the punch and die. The finished end wall on the skirt is produced by a shoulder on the punch or by any suitable abutment means associated with the punch.

The finished piston 15 shown in FIGURES 5 and 6 is formed from the blank of FIGURES 3 and 4 by machining including flash removal, cutting of the piston ring grooves 16 in the ring flange, drilling the wrist pin holes 17 in the pin bosse and cutting horizontal slots 17a through the skirt at the thicker transition area between the ring flange 13b and thinner skirt portion and normal to the pin bosses 13d.

The skirt portions 130 and 132 below or beyond the ring flange 13b are cam ground to produce an oval contour for the skirt with the major diameter of the oval extending normal to the wrist pin axis. The ring flange and the head of the piston remain in true circular form, and the cam grinding is confined to the outer face of the skirt. The interior of the piston need not be further finished. The corner 18 between the head and the ring flange and the steps 19 between the ring flange and the skirt, as well as the shoulder 20 between the skirt and its reduced diameter portion are accurate and well defined. The fillets 21 between the pin bosses and head, the rounded ends 22 of the pin bosses, and the finished recesses 23 in the scalloped end of the piston skirt are also smooth and well defined.

In accordance with this invention, as illustrated in FIG- URE 7, the piston head and ring flange portion of the piston 15 have the same diameter D through the center of the wrist pin holes 17 as well as at right angles to this diameter. The piston 15 is seated in the bore B of a cast.

iron cylinder C. The bore B, as illustrated, has a diameter equal to the diameter D of the piston plus the piston ring gap R spanned by piston rings seated in the grooves of the ring flange. The bore B hould be sufficiently greater than the diameter D to allow working clearance for the piston rings. A concentric seating of the piston head and ring flange portion of the piston in the cylinder C is provided.

As shown in FIGURE 8, the skirt portion 130 of the piston 15 is cam ground with a minor diameter D through the center of the Wrist pin holes 17 which may or may not be of the same diameter D through the ring flange portion, and a major diameter D normal to the wrist pin axis. The cylinder C has a constant diameter bore B along its length which is greater than the major diameter D of the skirt only enough to give a tight fit clearance for the working faces of the skirt which are normal to the wrist pin axis. As shown, in FIGURE 8, the bore B has a diameter which is from .001 to .002 inch longer than the major diameter D of the piston. As also shown, the major diameter D is from .01 to .025 inch longer than the minor diameter D, so that the bore will be from .0115 to .027 inch larger than the minor diameter D.

The diameters of the skirt disposed 45 from the major and minor diameters D and D are illustrated at D", and as shown, these diameters are intermediate the diameters D and D, so that the clearances will decrease from the illustrated clearances of .0115 to .027 at the minor diam eter to .0015 to .002 inch at the major diameter along a smoothly merging contour.

The relative dimensions of FIGURE 8 are provided for the initial cold fitted piston 15 of this invention. Heretofore, such tight fits at the major diameter of a piston skirt have not been possible without the aid of expansion control bands or struts of ferrous metal embedded in or anchored to the aluminum piston. Such clearances in a bandless piston have heretofore caused the piston to seize and scuff.

The aluminum composition and the grain band formation of the pistons of this invention have made possible the tight fitting of bandless pistons in a bore of a cylinder because the pistons of this invention are sufliciently resilieat and are capable of grain band flow to such an extent as to conform themselves to the cylinder bore at the elevated temperatures and stresses existing in operation of the engine. These stresses may reach peak loads of from 500 to 700 pounds per square inch on the piston head, while the operating temperatures may vary from 500 to 600 F. at the piston head down to 180 F. at the piston skirt. The pistons of this invention are heat treated prior to machining at temperatures of from 940 to 1000 F. for two to four hours followed by a quenching in Water and a precipitation hardening cycle involving temperatures between 350 to 450 for a period of from six to twenty hours is also desirable.

As illustrated in FIGURE 9, as the piston is heated in the engine and is subjected to the Working stresses of the engine, it will expand along its major diameter D to decrease the initial clearance fit up to zero, which means that the diameter D can have an expansion up to the diameter of the bore B. Before the expanded piston can seize or scuff the bore B, however, it will deform to flow metal from the tight spots along the major diameter D to the diameters D" displaced 45 from the major diameter. These diameters D will expand a distance equivalent to E or less than the full expansion E at the major diameter D. The minor diameter D may also slightly expand a distance E but most of the metal from the tight spots will flow into the areas along the diameters D". After operation in the engine for a break-in period of 20 to 25 hours, the piston skirt will permanently deform to the fitted configuration in the bore, as illustrated in FIGURE 9.

Deformation of the piston along the skirt occurs by sidewise flow of the grain bands outwardly from the major diameter area D as shown by the arrows 25 toward the diameters D". A circumferential flow of metal from the tight spots occurs by sidewise fiow of the grain bands without rupturing the bands.

From the above descriptions, it will therefore be understood that this invention provides extruded aluminum pistons with grain band formations in the skirt areas thereof which accommodate circumferential flow of metal under stress to make possible a tight fitting of the piston skirt in the bore of a cast iron cylinder especially along the working or slap faces of the skirt which are normal to the wrist pin axis. The metallurgical properties of the pistons of this invention impart resiliency to the initial piston, enabling it to conform with the bore of the cylinder in which it is seated and to then take a permanent set which will not accommodate further deformation. The pistons of this invention are made from low-expansion, high silicon content aluminum alloys, have a grain band configuration accommodating deformation, are tightly fitted in cylinders and will take a permanent set in conformity with the cylinder after a short break-in period of operation in the engine.

I claim as my invention:

1. A resilient slotted type aluminum piston free from expansion control bands and struts and having an oval shaped skirt with a major axis normal to the wrist pin axis thereof and sized for cold fits in a ferrous metal cylinder of from .001 to .002 inch clearance at the major diameter which comprises an extruded one piece aluminum alloy member composed by weight essentially of from 9% to 24% silicon, 1 /2 to 4% copper, .2% to 1% manganese, .2% to 1% magnesium, and balance aluminum, said member having a piston head portion, a ring band portion depending from-the head portion, a skirt portion depending from the ring band portion, and opposed wrist 'pin bosses depending from the head portion inside the ring band and skirt portions and integral therewith, said head portion having grain bands extending thereacross and radiating from the center thereof and thence through the ring band portion, wrist pin and skirt portions to define the interior and exterior surfaces of said portions with their sides, said skirt portion having a major diameter normal to the wrist pin bosses which is greater than the minor diameter through the wrist pin bosses by from .01 to .025 inch and which is sized for fitting in a cylinder with a diameter only from .001 to .002 inch greater than said major diameter, said piston characterized by a solution heat treated, quenched and precipitation hardened structure effected by a solution heat treatment at temperatures from 940 to 1000 F. for 2 to 4 hours, a water quenching, and a precipitation hardening at temperatures from 350 to 450 F. for a period of from six to twenty hours, and said piston in operation in the engine taking a permanent set to conform withthe cylinder bore by flow of the metal grain bands circumferentially from the major diameter of the skirt portion toward the minor diameter of the skirt portion.

2. A resilient slot-type piston composed of a low-expansion high silicon content solution heat-treated aluminum alloy having an oval shaped skirt with a major diameter normal to the wrist pin bosses sized for tight fitting in an engine cylinder with a clearance of from .001 to .002 inch and having metal grain bands extending axially through the skirt to define the exterior oval surface of the skirt with their sides only, said metal grain bands being deformable to flow from the major diameter toward the minor diameter of the skirt under temperatures and stress existing in the skirt area during normal operation of the engine, and said skirt becoming permanently set in fitting conformity with the cylinder bore after a break-in operation in the engine.

3. A bandless aluminum piston adapted to take a permanent set upon operation in an engine and free from expansion control means which comprises a one piece aluminum trunk-type piston with a cylindrical head and ring band portion and an oval skirt having a major diameter normal to the Wrist pin axis thereof, said piston having aluminum metal grain bands radiating from the head through the ring band portion and skirt to terminate at the free end of the skirt and define the side walls of the skirt with their side faces only, said aluminum having a solution heat treated structure and deformable being under stress and load existing in the operating engine to flow metal from the major diameter of the skirt toward the minor diameter and thence assume a permanently set con-- dition conforming with the cylinder contour.

4. A piston in cylinder assembly which comprises an extruded slot-type low-expansion solution heat-treated. aluminum alloy piston having an oval skirt with a major diameter normal to the wrist pin axis of the piston, a cylinder having a cylindrical bore receiving the piston, said bore having a diameter of from .001 to .002 inch greater than said major diameter of the skirt, the aluminum of said piston having elongated grain bands extending across its head and through the skirt to terminate at the end edge of the skirt, and sai grain bands being bendable under stresses and temperatures existing in the cylinder to conform the skirt to the bore of the cylinder without seizing or scuffing the cylinder.

5. An internal combustion engine cylinder and piston assembly which comprises a ferrous metal cylinder having a cylindrical bore, a piston mounted in said bore having a circular head and ring band of less diameter than the bore together with an oval skirt with a major diameter normal to the wrist pin axis of the piston and sized from .001 to .002 inch less than the diameter of the bore, said oval skirt having an axial aluminum metal grain band formation adapted to flow circumferentially and radially under stress and temperature conditions existing in the engine during operation of the piston to conform the skirt to the bore of the cylinder, said piston being composed of a low expansion aluminum alloy, with a solution heat treated structure created at temperatures from 940 to 1000 F. for two to four hours and assuming a permanent set in conformity with the cylinder after a break-in operation in the engine.

6. The method of making bandless aluminum pistons which comprises casting a cylindrical slug of low-expansion aluminum alloy, extruding the slug into trunk piston configuration, solution heat treating the piston, precipitation hardening the piston, machining the piston to an oval skirt configuration with a major diameter normal to the Wrist pin axis of from .001 to .002 inch less than the diameter of a bore to receive the piston, operating the piston in an engine, and conforming the shape of the skirt to the shape of the bore as the piston assumes a permanent set under the stress and temperatures of engine operation.

7. The method of making bandless aluminum pistons which comprises casting a cylindrical slug of low-expansion aluminum alloy, extruding the slug into trunk piston configuration, solution heat treating and precipitation hardening the piston producing an unset piston, machining the piston to an oval skirt configuration with a major diameter normal to the wrist pin axis of from .001 to .002 inch less than the diameter of a bore to receive the piston, operating the unset piston in an engine for to hours and thereby reshaping the unset piston causing flow metal grain bands circumerentially from the major diameter of the oval skirt toward the minor diameter of the oval skirt to conform the skirt with the shape of the cylinder bore without seizing or sufiing the cylinder bore with the piston then assuming a permanent set under the stress and temperatures of engine operation.

8. The method of making a resilient type aluminum piston free from expansion bands and struts capable of taking a permanent set to conform with a cylinder bore by flow of the metal grain band circumferentially from the major diameter of the skirt toward the minor diameter of the skirt during the operation of the piston in an engine which comprises the steps of heat- .ing a cast solid slug of a high silicon content aluminum alloy to forging temperatures notexceeding 950 F. impact forging the slug endwise to substantially decrease the height of the slug and to increase the diameter of the .slug, hot forging the resulting billet obtained from the .slug at forging temperatures not exceeding about 950 F., and extruding wrist pin bosses, extruding the slug into trunk piston at temperatures from 940 to 1000 F. from 2 to 4 hours, precipitation hardening the piston at temperatures from 350 to 450 for a period from 2 to 20 hours, machining the piston to an oval skirt configuration with a major diameter normal to the wrist pin axis from .001 to .002 inch less than the diameter of a bore to receive the piston, and inserting the piston into cylinder bore normally operating an engine having the piston therein and thereby reshaping the piston to conform with the cylinder bore by causing flow of the metal grain bands circumferentially from the major diameter of the skirt toward the minor diameter of the skirt with the piston then taking a permanent set.

9. The method of making a resilient slotted type aluminum piston free from expansion bands and struts capable of taking a permanent set to conform with a cylinder bore by flow of the metal grain band circumferentially from the major diameter of the skirt toward the minor diameter of the skirt during the operation of the piston in an engine which comprises casting a cylindrical slug of aluminum alloy composed by weight essentially of from 9% to 24% silicon, 1 /2 to 4% copper, .2% to 1% manganese, .2% to 1% magnesium, and balance aluminum, extruding the slug into trunk piston configuration with wrist pins, solution heat treating the piston at temperatures from 940 to 1000 F. from 2 to 4 hours a precipitation hardening the piston at temperatures from 350 to 450 for a period from 6 to 20 hours, machining the piston to an oval skirt configuration with a major diameter normal to the wrist pin axis from .001 to .002 inch less than the diameter of a bore to receive the piston, and operating the piston in an engine and reshaping the piston to conform with the cylinder bore by causing fiow of the metal grain bands circumferentially from the major diameter of the skirt toward the minor diameter of the skirt with the piston than taking a permanent set.

References Cited in the file of this patent UNITED STATES PATENTS 1,835,863 Grene Dec. 8, 1931 2,024,285 Handler Dec. 17, 1935 2,217,542 Flamming et a1. Oct. 8, 1940 2,344,353 King Mar. 14, 1944 2,413,154 Porter Dec. 24, 1946 2,465,792 Davis Mar. 29, 1949 2,497,380 Venner et al Feb. 14, 1950 2,513,814 Moore Apr. 4, 1950 2,667,390 Watson et al. Jan. 26, 1954 2,771,327 Reinberger Nov. 20, 1956 2,795,467 Colwcll June 11, 1957 2,869,946 Herrmann Jan. 20, 1959 2,963,329 Schrerenberg Dec. 6, 1960 

1. A RESILIENT SLOTTED TYPE ALUMINUM PISTON FREE FROM EXPANSION CONTROL BANDS AND STRUTS AND HAVING AN OVAL SHAPED SKISRT WITH A MAJOR AXIS NORMAL TO THE WRIST PIN AXIS THEREOF AND SIZED FOR COLD FITS IN A FERROUS METAL CYLINDER OF FROM .001 TO .002 INCH CLEARANCE AT THE MAJOR DIAMETER WHICH COMPRISES AN EXTRUDED ONE PIECE ALUMINUM ALLOY MEMBER COMPOSED BY WEIGHT ESSENTIALLY OF FROM 9% TO 24% SILICON, 1 1/2 TO 4% COPPER, 2% TO 1% MANGANESE, .2% TO 1% MAGNESIUM, AND BALANCE ALUMINIUM, SAID MEMBER HAVING A PISTON HEAD PORTION, A RING BAND PORTION DEPENDING FROM THE HEAD PORTION, A SKIRT PORTION DEPENDING FROM THE RING BAND PORTION, AND OPPOSED WRIST PIN BOOSES DEPENDING FROM THE BASED PORTION INSIDE THE RING BAND AND SKIRT PORTIONS AND INTEGRAL THEREWITH, SAID HEAD PORTION HAVING GRAIN BANDS EXTENDINNG THEREACROSS AND RADIATING FROM THE CENTER THEREOF AND THENCE THROUGH THE RING BAND PORTION, WRIST PIN AND SKIRT PORTIONS TO DEFINE THE INTERIOR AND EXTERIOR SURFACES OF SAID PORTION WITH THEIR SIDES, SAID SKIRT POSRTION HAVING A MAJOR DIAMETER NORMAL TO THE WRIST PIN BOSSES WHICH IS GREATER THAN THE MINOR DIAMETER THROUGH WRIST PIN BOSSES BY FROM .01 TO .025 INCH AND WHICH SIZED FOR FITTING IN A CYLINDER WITH A DIAMETER ONLY FROM .001 TO .002 INCH GREATER THAN SAID MAJOR DIAMETER, SAID PISTON CHARACTERIZED BY A SOLUSTION HEAT TREATED, QUENCHED AND PRECIPITATION HARDENED STRUCTURE EFFECTED BY A SOLUTION HEAT TREATMENT AT TEMPERATURES FROM 940 TO 1000*F. FOR 2 TO 4 HOURS, A WATER QUENCHIONG, AND A PRECIPITATION HARDENING AT TEMPERATURES FROM 350* TO 450* F. FOR A PERIOD OF FROM SIX TO TWENTY HOURS, AND SAID PISTON IN OPERATION IN THE ENGINE TAKING A PREMANENT SET TO CONFORM WITH THE CYLINDER BORE BY FLOW OF THE METAL GRAIN BANDS CIRCUMFERENTIALLY FROM THE MAJOR DIAMETER OF THE SKIRT PORTION TOWARDS THE MINOR DIAMETER OF THE SKIRT PORTION. 